For a piezoelectric ink jet head that uses the electrostrictive effect of a piezoelectric element as the drive power source and is employed in an on-demand type ink jet printer, one having such a constitution is widely employed that comprises a plurality of pressure chambers to be filled with an ink disposed on one side of a plate-shaped substrate along the surface, with a nozzle for discharging the ink provided to communicate with each of the pressure chambers and a drive section including the piezoelectric element provided for each of the pressure chambers, as described in Japanese Unexamined Patent Publication JP-H-05-318731-A2 (1993).
In the piezoelectric ink jet head described above, a drive voltage is individually applied to one or more of the piezoelectric elements each corresponding to each of the pressure chambers so as to deform, thereby decreasing the volume of the pressure chamber that corresponds to the piezoelectric element, so that the ink contained in the pressure chamber is discharged from the nozzle that communicates therewith in the form of ink droplet and a dot is formed on a sheet of paper.
More specifically, a drive section comprising the piezoelectric element and an oscillator plate that supports the piezoelectric element transmits a force generated by the piezoelectric element as a pressure to the ink contained in the pressure chamber, thereby to function as a drive power source that discharges ink droplets through the nozzles that communicate with the pressure chambers. That is, the drive section causes the piezoelectric element to deform due to the drive voltage applied thereto, so that the oscillator plate is caused to deflect and protrude toward the pressure chamber, thereby decreasing the volume of the pressure chamber and pressurizing the ink in the pressure chamber, so that an ink droplet is discharged from the tip of the nozzle.
At the same time, since the oscillator plate is caused by the pressure of the ink contained in the pressure chamber to deflect in a direction opposite to that described above, the drive section also acts as an elastic body with respect to the vibration of the ink in the head.
When a drive voltage is applied to the piezoelectric element so as to generate a force, the ink contained in the head undergoes vibration under the pressure transmitted via the oscillator plate from the drive section. This vibration is generated as the drive section and the pressure chamber act as the elasticity against the inertia of a feeder port that feeds the ink to the pressure chamber, a nozzle passage that communicates with the pressure chamber and the nozzle, and the nozzle. Natural period of vibration of volumetric velocity of the ink contained in the head during this vibration is determined by the dimensions of the components described above, physical properties of the ink and dimensions and physical properties of the drive section.
In the piezoelectric ink jet head, an ink droplet is discharged by utilizing the vibration of ink meniscus in the nozzle due to the vibration of the ink described above, thereby forming a dot on the paper surface.
In order to achieve a higher resolution of the piezoelectric ink jet head while decreasing the size of the piezoelectric ink jet head, pitch of arranging the nozzles must be made as small as possible. When resolution of the piezoelectric ink jet head becomes higher and the number of nozzles increases, however, it becomes difficult to dispose the independent piezoelectric elements individually in correspondence to the pressure chambers. For this reason, it has recently become a prevailing practice to employ a piezoelectric ink jet head having a piezoelectric element made in a thin plate of transverse vibration mode that is formed integrally with an electrode (common electrode), lower (oscillator plate side) one of a pair of electrodes that are disposed to sandwich the piezoelectric element for applying the drive voltage to the piezoelectric element, and the oscillator plate, in such a size that covers the plurality of pressure chambers (hereinafter referred to as a “common element type”). Of the pair of electrodes, the electrode that is disposed over the piezoelectric element (individual electrode) is separately formed in a predetermined shape that corresponds to each pressure chamber for applying drive voltage individually to each piezoelectric element.
In the piezoelectric ink jet head of common element type, when an electric field is generated by applying the drive voltage from the individual electrode to the region sandwiched by the individual electrode and the common electrode in the plane of the piezoelectric element (hereafter referred to as a “drive region”), the drive region can be driven like an independent piezoelectric element thereby pressurizing the ink in the corresponding pressure chamber.
In case adjacent pressure chambers are disposed too close to each other, however, there arises such a problem that, when the piezoelectric element (drive region in the case of common element type) that corresponds to a particular one of the pressure chambers is driven, the piezoelectric element (or the drive region) corresponding to the adjacent pressure chambers that surround the particular pressure chamber are subject to the influence of driving of the piezoelectric element. This problem is conspicuous particularly in the piezoelectric ink jet head of common element type.
Specifically, in the piezoelectric ink jet head of common element type, the piezoelectric element 9 made in a thin plate of transverse vibration mode is fixed at the upper end face of a partition wall 1e of the substrate 1, that separates the adjacent pressure chambers 2, via the common electrode 8 and the oscillator plate 7, whereby the piezoelectric element 9 has such a structure as the non-driving region fixed on the upper end face of the partition wall 1e constrains the periphery of the drive region that is defined by the planar configuration of the individual electrodes 10, as shown in FIG. 6.
When the drive voltage is applied to the individual electrode 10 located at the second position from the far left in the figure so as to drive the drive region of the piezoelectric element 9 that corresponds to the above-mentioned individual electrode 10 thereby causing the drive region, the common electrode 8 that is disposed right under thereof and the oscillator plate 7 to deform in the direction indicated by the hollow arrow (hereafter referred to as a “positive direction”) in FIG. 7 so that volume of the pressure chamber 2 decreases and the ink is discharged, then the drive regions of the oscillator plate 7, the common electrode 8 and the piezoelectric element 9 located above the pressure chambers 2 located on both sides of the above-mentioned pressure chamber 2 deform in the direction of increasing the volume of the pressure chamber 2 (hereafter referred to as a “negative direction”) as indicated by black arrow, although the figure shows only one side.
As a result, under the condition shown in FIG. 7, when the drive voltage is further applied to the individual electrodes 10 located above the adjacent pressure chambers 2 that have deformed in the negative direction so as to drive the drive region of the piezoelectric element 9 corresponding to the individual electrode 10, the amount of deformation in the positive direction becomes the amount of deformation in the positive direction, achieved by independently driving the drive region in question without driving the drive regions on both sides thereof, minus the amount of deformation in the negative direction described above. In consequence, pressure applied via the oscillator plate 7 to the ink in the pressure chamber 2 right below thereof for discharging the ink decreases accordingly, resulting in a smaller ink droplet discharged from the nozzle and/or a lower discharging speed that causes smaller dot size and/or deformed dot shape.
This phenomenon occurs simultaneously in a plurality of drive regions that are disposed in the planar direction of the piezoelectric element 9 of thin-plate construction.
In case the drive regions on both sides of one drive region of the piezoelectric element 9 are driven and, under this condition, the drive region interposed therebetween is driven, the amount of deformation of the drive region in the positive direction becomes even smaller than that of the case described above.
State of driving a particular drive region is also under influence of the state of driving the drive regions located beyond the adjacent drive regions, or the state of driving other neighboring drive regions, for example, although the magnitude of the influence is far smaller than that of the adjacent drive regions.
Thus when attention is focused on a particular drive region, the amount of deformation of the drive region in the positive direction varies depending on the state of driving the plurality of surrounding drive regions, namely the dot pattern of the picture to be formed, resulting in variability in dot size and/or dot shape.
The phenomenon described above occurs similarly also in the conventional type (hereafter referred to as a “separated element type”) where separate piezoelectric elements are provided for the individual pressure chambers. In this case, deformations of the oscillator plate and of the common electrode disposed thereon influence the piezoelectric element disposed on the adjacent pressure chambers.